Many laser applications involve knowledge that is as accurate as possible with respect to beam parameters such as e.g. the beam size, beam decentration, beam inclination or beam divergence, and also involve the correction of aberrations (such as e.g. astigmatism, coma and spherical aberration).
An issue occurring here in practice is that, e.g., thermally induced wavefront changes or aberrations of the laser beams may occur, knowledge about which that is as exact as possible being involved for a correction that takes place during the operation (in real time).
However, in this case, the use of sensors that are conventionally used for wavefront measurement (such as, e.g., so-called Shack-Hartmann sensors with a CCD camera situated in the focal plane of the microlens arrangement) only has restricted suitability to the extent that, on account of the geometric reference centers (e.g., vertices or apertures of the lenses in the microlens arrangement in the case of a Shack-Hartmann sensor) that have been introduced, as a matter of principle, by the measurement arrangement, the respective measurement result is also influenced by effects which are based on the interaction or shearing of the coordinate system of the laser radiation on the one hand with the coordinate system of the measurement arrangement on the other (such that the intrinsic coordinate system inherent to the measurement arrangement is virtually “impressed” onto the measurement result). This has as a consequence, in particular, that beam disturbances occurring during the measurement, for instance as a consequence of a positional change of the measurement arrangement relative to the laser beam, are immediately noticeable in the measurement result and, in this respect, make a reliable wavefront analysis more difficult, or prevent the latter, since it is not determinable whether a measured wavefront effect is based on an actually occurred (e.g. thermally induced) wavefront modification or only based on a positional change (e.g. “wobbling”) of the measurement arrangement relative to the laser beam.
Expressed differently, there is a need during the analysis of wavefront aberrations of a laser beam to ascertain the wavefront aberrations in the coordinate system of the laser beam itself (and not in that of the measurement arrangement).
Moreover, there is also a need to obtain the corresponding results sufficiently quickly during the operation of the respective system in order to be able to promptly correct possible wavefront aberrations, for instance using an adaptive mirror or the like.
A purely exemplary laser application is the laser plasma source which is used in e.g. lithography for producing EUV light (e.g. at wavelengths of e.g. approximately 13 nm or approximately 7 nm) and with respect to which FIG. 12 shows a schematic illustration of the possible construction. The EUV laser plasma source according to FIG. 12 has a high-energy laser (not shown here) e.g. for generating infrared radiation 6 (e.g. a CO2 laser with a wavelength of λ≤10.6 μm), the infrared radiation being focused by way of a focusing optical unit, passing through an opening 11 present in a collector mirror 10 embodied as an ellipsoid and being guided onto a target material 32 (e.g. tin droplets) which is generated via a target source 35 and supplied to a plasma ignition position 30. The infrared radiation 6 heats the target material 32 situated in the plasma ignition position 30 in such a way that the target material transitions into a plasma state and emits EUV radiation. This EUV radiation is focused by way of the collector mirror 10 onto an intermediate focus IF and enters through the latter into a downstream illumination device, the edge 40 of which is merely indicated and which has a free opening 41 for the light entrance.
Both the droplet position of the (e.g., tin) droplets forming the target material and the focus position of the laser beams to be tracked accordingly can be determined using a so-called beam propagation camera, wherein both the laser beams in the “forward direction” (the infrared radiation 6 prior to incidence on the respective target droplets) and the laser beams in the “backward direction” (the infrared radiation 6 reflected back from the respective target droplet) are detected and the measurement data involved for the laser beam guidance and droplet guidance are obtained. Here, there is a need to be able to promptly correct thermally induced aberrations, involving an accurate and fast analysis of the laser beams.
With regard to the prior art, reference is made by way of example to WO 2015/113713 A1.